Hydraulic fracturing is a technique of fracturing rock formations by a pressurized fluid in order to extract oil and natural gas contained in the formations. A fluid, which usually is water mixed with sand and chemicals, is injected into a wellbore under considerable pressure to create fractures in the formations. When the pressure is removed from the wellbore, the sand props the fractures open allowing the oil and gas contained in the formations to more readily flow into the well for extraction. This technique has revolutionized oil and gas development, especially is shale formations, because it permits extraction of formerly inaccessible hydrocarbons. As a result, it has helped push U.S. oil production to a new high and generate billions of revenues to mineral rights owners, oil companies, as well federal, state, and local governments.
Hydraulic fracturing, however, can be a very expensive process, especially if the quality of the formations is unknown. In a horizontally drilled oil well, hydraulic fracturing generally is performed in several stages along the horizontal portion of the well. Typically, the horizontal portion of the well is stimulated in stages about every 200 to 250 feet. Although the horizontal portion of the well generally extends through a given hydrocarbon bearing formation, the lithology or rock quality may vary along the length of the wellbore. When oil companies conduct a frac treatment at a section of the formations that is sub-optimal, the stimulation may be ineffective or produce marginal gains in productivity for that particular stage. Assuming that the average cost for each hydraulic fracture treatment is approximately $100,000 and that some formations may have up to 80% of its sections be sub-optimal, the cost and time spent in fracturing sub-optimal sections or in determining whether to move onto another section can be substantial. In one year, an energy consulting company estimated that about $31 billion was spent in sub-optimal fracturing across 26,100 U.S. oil wells.
Moreover, even if the oil drilling companies treat a section of the formation that happens to be optimal, the treatments may not have been the optimal size. In other words, the treatment may have been too small given the favorable rock qualities that existed for that particular stage and that the well could have been even more productive and the return on the investment of the stimulation could have been even higher had a larger stimulation been pumped, or had a different stimulation fluid or amount of proppant been pumped. As such, knowing the quality of the formations prior to a hydraulic fracture treatment is beneficial to stimulation treatments.
A method called Distributed Fiber Optic Sensing has been developed to provide this information. This method is based on either temperature or acoustic sensing. In the method based on temperature sensing, a unit including a laser source and a photodetector is placed on the surface and a glass fiber is permanently installed in the well. The laser source sends laser pulses down the glass fiber and the temperature of the formations can affect the glass fiber and locally change the characteristics of light transmission in the glass fiber. The photodetector measures the laser light reflections from different spots in the glass fiber due to the temperature and the spectrum of the laser light reflections can used to determine the properties of the formations. The method based on acoustic sensing is similar to the temperature sensing one except that this method employs a unit that includes an acoustic signal generator and an acoustic signal receiver and that this method measures the reflected acoustic signals based on the strain or pressure of the formations exerted on and along various points of the glass fiber. The measured acoustic signals may have various amplitude, frequency, and phase attributes that can also be used to determine the properties of the formations.
The Distributed Fiber Optic Sensing method, however, has several drawbacks. First, this method requires running a glass fiber into the well that complicates the installation process. Second, this method usually costs around $600,000 to implement and the investment is only for one single well and is permanent. Third, this method is not economically practical on smaller reservoir wells. Fourth, to protect the fragile glass fiber, the glass fiber is typically placed within a stainless steel sheath that can attenuate the temperature or strain response, reducing accuracy of the measurement.
Accordingly, there is a need for an improved method for determining the quality of the rock formations prior to a hydraulic fracture treatment.